On Wed, 4 Mar 2009, Luigi Rizzo wrote:
> On Wed, Mar 04, 2009 at 08:17:05PM +0100, Sebastian Mellmann wrote:
> > Hi everyone!
> >
> > I hope this is the right place to ask.
> >
> > I've got a IPFW ruleset that looks like this:
> >
> > cmd=ipfw
> > bottleneck_bandwidth=100Mbit/s
> > in_if="em0"
> >
> > $cmd pipe 500 config bw $bottleneck_bandwidth
> > $cmd add pipe 500 all from any to any via $in_if
>
> the delay that a packet experiences corresponds to len/bandwidth,
> often rounded up to the next clock tick (1ms is the default).
> You get one delay inbound, one delay outbound, so that's 2ms.
After finally getting almost enough sleep, I've just realised, duh, that
Sebastian likely already had the default kern.hz=1000, ie 1ms, so would
need something faster to achieve less delay.
Which led me to take my own medicine and reread the dummynet sections in
ipfw(8) at 7.1-RELEASE:
delay ms-delay
Propagation delay, measured in milliseconds. The value is
rounded to the next multiple of the clock tick (typically 10ms,
but it is a good practice to run kernels with ``options HZ=1000''
to reduce the granularity to 1ms or less). Default value is 0,
meaning no delay.
Firstly, this is well out of date; the default has been HZ=1000 since
6.1-R or earlier, a ten-fold increase on the old 100Hz. I'm not sure
however that 10000 would be a suitable suggestion, even with today's
processor speeds?
Secondly, apropos Sebastian's experience, should this say "The value
(even if 0) is rounded to the next multiple of the clock tick .." ?
^^^^^^^^^^^
cheers, Ian
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